Field of Endeavor
Embodiments of this disclosure relate generally to the field of passive optical networks where the return path is a multipoint-to-point optical path where multiple optical signals are detected by a single optical receiver. This disclosure relates to transport of CATV (cable television) and digital signals over optical components, both passive (optical filters, optical fiber, etc.) and active that are detected by a single optical receiver.
Discussion of Related Art
A large number of optical paths are combined and then connected to a single receiver in the return path of a multipoint-to-point optical network. If two or more optical signals are simultaneously present in the return path of such networks, and their optical wavelengths happen to cross each other (resulting in an optical wavelength collision), then optical heterodyning between the signals involved in the optical collisions results in a very large increase in the noise floor, a phenomenon referred to as Optical Beat Interference (OBI).
OBI is avoided in typical Passive Optical Networks (PONs) since they usually operate in a Time Division Multiplexing Access (TDMA) scheme whereby only one return path optical source is allowed to be on at any given time. However, there are some multipoint-to-point systems, such as Radio Frequency Over Glass (RFoG) systems in CATV networks, where it is possible that multiple return path optical sources are simultaneously on. This happens, for example, if there are multiple services operating (such as telephone, high-speed data, etc), each of which allows one optical source to be on. A second way that multiple optical sources could be on simultaneously in the return path is that DOCSIS (data over cable service interface specifications) systems in CATV networks allow multiple return path transmitters to be operating simultaneously, in different frequency bands. This means that OBI could occur in such networks, with a resulting possibility of service interruptions.
One method that manufacturers have used to avoid OBI in susceptible networks is to use active or passive techniques to avoid the possibility of two return path wavelengths being too close to each other. One passive method of avoiding OBI is to sort the optical transmitters into different “wavelength bins” that are wide enough so that transmitters in different bins do not beat with each other over typical operating conditions—but this means using many different transmitter sort sub-sets (each with a corresponding part number) and expending much effort in sorting transmitters. This would reduce effective transmitter yield in context because some transmitters would not fit into any of the wavelength bins. The maximum number of splits in such a system would also be limited by the number of wavelength bins available. Supporting PON systems with 64 or 256 splits would be difficult in such a passive OBI avoidance technique.
An active method of avoiding OBI is to actively tune the transmitter wavelength (through relatively low-cost methods such as temperature tuning) in order to avoid OBI. This necessarily entails higher cost, as more robust TECs (thermoelectric coolers) must be employed for temperature (and hence wavelength) control. A managed system could also be used whereby communications with the optical transmitter allows the operator to actively change the transmitter wavelength.
Disadvantages of both active and passive techniques of controlling transmitter wavelengths are that costs are higher, and also, the operator must only use that particular brand of transmitter. The operators are not able to add other wavelengths from other vendors, since then OBI might occur. Also, some customers might desire an OBI avoidance technique that works for any set of return path wavelengths, even if some of them are identical. One trivial method of doing this is to use one receiver for each return path transmitter, then combine the signals in the electrical domain, then re-modulate another transmitter with the combined RF (radio frequency) signal. This OBI-avoidance technique, however, would be too expensive (especially for large number of split such as 64, 128 or 256-way splits) and would be unable to compete with other solutions that did not entail the use of a large number of receivers.